1. Field of the Invention
The present invention relates generally to coaxial terminations used to terminate ports that are adapted to receive coaxial cable connectors, and more particularly, to an improved coaxial termination that offers protection against high-voltage surges.
2. Description of the Related Art
RF coaxial cable systems are well known to those in the cable television industry for distributing radio frequency signals to subscribers of cable television service, and more recently, voice and data telecommunications services. The coaxial cables used to route such signals include a center conductor for transmitting a radio frequency signal, and a surrounding, grounded outer conductive braid or sheath. Typically, the coaxial cable includes a dielectric material surrounding the center conductor and spacing it from the grounded outer sheath. The diameter of the center conductor, and the diameter of the outer conductor, and type of dielectric are selected to produce a characteristic impedance, such as 75 ohms, in the coaxial line. This same coaxial cable is sometimes used to provide AC power (typically 60-90 Vrms) to the equipment boxes that require external power to function. Approximately 80% of the cable in a system will carry this AC power.
Within such coaxial cable systems, such coaxial lines are typically coupled at their ends to equipment boxes, such as signal splitters, amplifiers, etc. These equipment boxes often have several internally-threaded coaxial ports adapted to receive end connectors of coaxial cables. If one or more of such coaxial ports is to be left xe2x80x9copenxe2x80x9d, i.e., a coaxial cable is not going to be secured to such port, then it is necessary to xe2x80x9cterminatexe2x80x9d such port with a coaxial termination that matches the characteristic impedance of the coaxial line (e.g., a 75 ohm termination). If such a coaxial termination is omitted, then undesired reflected signals interfere with the proper transmission of the desired radio frequency signal.
Coaxial terminations of the type described above are known and available. Typically, such known coaxial termination devices include a metallic outer body which, at a first end thereof, is provided with external threads for mating with the internal threads of a coaxial port on the equipment box. A center conductor passes through a dielectric secured within the metallic outer body from the first end of the coaxial termination device to an opposing second end thereof. At the second end of the coaxial termination device, a resistor corresponding to the characteristic impedance of the coaxial line is secured, and is coupled between the center conductor and the grounded metallic outer body. If the coaxial line carries AC or DC power, then a low frequency blocking capacitor is typically used to couple the aforementioned resistor to ground. The resistor and capacitor of such known coaxial termination devices are often located outside the controlled characteristic impedance environment, creating an impedance mismatch that reflects some of the forward-transmitted signal back toward its source. These reflections can result in loss of power transfer and interference with, or corruption of, the signal. Accordingly, some signal degradation results from the use of such coaxial termination devices. The degree of such signal degradation at a given frequency, resulting from such impedance mismatch, is sometimes expressed as the RF return loss performance of the coaxial system.
Moreover, when deployed in the field, as in cable TV systems, for example, these known coaxial termination devices can be subjected to power surges caused by lightening strikes and other events. These power surges can damage or destroy the resistive and/or capacitive elements in such a termination, rendering it non-functional. A commonly used surge test, ANSI C62.41 Category B3, specifies that a 6,000 Volt open circuit/3,000 Amp short circuit surge pulse be injected into the coaxial termination device. At least some of the known coaxial termination devices have difficulty complying with such surge test. Indeed, efforts to make the resistive and capacitive components larger, in order to withstand such power surges, can have the negative impacts of increased costs and/or creating a larger impedance mismatch, and hence, causing poorer levels of RF Return Loss performance. One approach to designing a termination that can withstand the previously mentioned 6,000 Volt surges would be to use a 6,000 Volt capacitor and a high power resistor. Unfortunately, such components are relatively expensive and have a much larger physical size, which tends to increase the size and cost of the housing necessary to contain such components, thereby resulting in a much bulkier and more costly design.
Accordingly, it is an object of the present invention to provide a coaxial termination device capable of maintaining high levels of RF Return Loss performance.
It is a further object of the present invention to provide such a coaxial termination device capable of withstanding power surges without damage to the resistive and/or capacitive elements thereof.
A further object of the present invention is to provide such a coaxial termination device that can simultaneously withstand such power surges without damage, while still maintaining high levels of RF Return Loss performance.
A still further object of the present invention is to provide such a termination device that is relatively compact and inexpensive to manufacture.
Another object of the present invention is to provide such a coaxial termination device that reduces reflection by disposing the resistive component thereof in a controlled characteristic impedance environment.
Still another object of the present invention is to minimize the length of the path between the resistive component of the coaxial termination device and ground (i.e., through the capacitive component) to further minimize inductance and signal reflection.
Yet another object of the present invention is to provide such a coaxial termination device which allows the resistive and capacitive components thereof to be relatively small in size to maintain high levels of RF Return Loss performance while still being able to withstand power surges without damage.
These and other objects of the present invention will become more apparent to those skilled in the art as the description of the present invention proceeds.
Briefly described, and in accordance with the preferred embodiments thereof, the present invention relates to a surge-protected coaxial termination that includes a metallic outer body having a central bore extending therethrough, a center conductor extending into the central bore of the metallic outer body, and a spark gap created within such coaxial termination for allowing a high-voltage power surge to discharge across the spark gap without damaging other components (e.g., resistive and/or capacitive components) that might also be included in such coaxial termination. The central bore of the outer body is bounded by an inner wall, and the center conductor has an outer diameter facing the inner wall of the outer body. Normally, there is a solid dielectric material separating the outer diameter of the center conductor from the inner wall of the outer body; however, in the vicinity of the aforementioned spark gap, the dielectric material is simply air or another ionizable gas.
In a first embodiment of the present invention, the spark gap is created by including an inwardly-directed step upon the inner wall of the outer body. This inwardly-directed step portion of the inner wall is of relatively short axial length and has an inner diameter that is significantly smaller than the inner diameter of the remainder of such inner wall of the outer body. The center conductor extends through the inwardly directed step of the inner wall; at the point where the center conductor passes through the inwardly-directed step, its outer diameter is slightly less than the inner diameter of the inwardly-directed step. This positions the inwardly-directed step of the inner wall in close proximity to the center conductor to form the spark gap therebetween. If desired, the outer diameter of the center conductor can be enlarged somewhat to form an outwardly-directed step at the point where it passes through the inwardly-directed step to facilitate the passage of a spark between the outwardly-directed step of the center conductor and the inwardly-directed step of the outer body.
In a second embodiment of the present invention, the surge-protected coaxial termination again includes a metallic outer body having a central bore extending therethrough, and a center conductor extending into the central bore thereof, but the spark gap is created by forming an outwardly-directed step of relatively short axial length on the center conductor extending toward the inner wall of the outer body. The outer diameter of the outwardly-directed step is slightly less than the inner diameter of the inner wall for positioning the outwardly-directed step of the center conductor in close proximity to the inner wall of the outer body to form a spark gap therebetween.
In a third embodiment of the present invention, the surge-protected coaxial termination again includes a metallic outer body having a central bore extending therethrough, and a center conductor extending into the central bore thereof, but the spark gap is created by a lateral conductor, such as a post or the like. This lateral conductor can be secured to the outer body and extend laterally toward the center conductor, or the lateral conductor can be secured to the center conductor and extend laterally toward the inner wall of the outer metallic body. In either case, the lateral conductor creates a spark gap that can discharge to ground any high voltage surges that appear between the center conductor and the outer conductor.
The creation of the spark gap in the manner described above tends to present a highly-capacitive discontinuity to any RF fields traveling along the transmission line; such a capacitive discontinuity would ordinarily cause reflections of the type that a coaxial termination device is designed to prevent. Accordingly, in the preferred form of the present invention, at least one relatively high characteristic impedance inductive zone is formed adjacent the capacitive spark gap; preferably, such high characteristic impedance inductive zones are formed on both sides of the spark gap. The combination of the capacitive spark gap and the high impedance inductive zones form the equivalent of an electrical T-network low pass filter, wherein the additional inductance of the high impedance zones effectively nullifies the additional capacitance of the spark gap, over the bandwidth of interest.
As mentioned above, coaxial termination devices typically include a resistive component to absorb the RF signal, and prevent the reflection of the RF signal. Accordingly, the preferred embodiments of the present invention include a resistive terminating element electrically coupled between the center conductor and the metallic outer body. This resistor is electrically in parallel with the spark gap, whereby surge currents that jump the spark gap flow around the resistor, avoiding damage thereto. Accordingly, the resistor can be relatively compact and inexpensive.
As also mentioned above, coaxial termination devices typically include an AC/DC power blocking capacitor coupled in series with the resistor between the center conductor and the metallic outer body. Once again, the capacitor can be relatively small and inexpensive because the spark gap protects the capacitor from damaging high voltage power surges.
Another novel feature of the preferred form of the present invention relates to the manner by which such resistive and capacitive components of the coaxial termination device are incorporated therein. Preferably, the resistive component is disposed inside the metallic outer body, and extends co-axially with the center conductor. Ideally, this resistive component is formed inexpensively as a carbon composition resistor. The resistive component may be surrounded by, and supported by, dielectric material disposed inside the central bore of the metallic outer body, thereby maintaining the resistor in a controlled characteristic impedance environment. One end (electrode) of the resistive component is electrically coupled with an end of the center conductor. The opposing second end (electrode) of the resistive component may protrude from the metallic outer body and related dielectric material; the DC blocking capacitor preferably extends radially between the second end of the resistive component and the metallic outer body, or to a grounding post secured thereto. Since the DC blocking capacitor is surge-protected, it may be of a compact and inexpensive design, such as a chip capacitor.
Another aspect of the present invention relates to such a device that is used to couple together two coaxial transmission devices, rather than to terminate a transmission path, while retaining the advantages of providing surge protection. This coupling device uses a similar outer body, center conductor, and spark gap as the aforementioned termination device; in the preferred form of the surge-protected coupler, relatively high characteristic impedance inductive zones are formed adjacent the capacitive spark gap on opposing sides thereof.